Chemical or electrochemical sensors and, in particular, gas sensors for gaseous media, work only inadequately outside their nominal, i.e., optimal measuring range. The respective sensors can also be damaged outside this nominal measuring range, which must be prevented. On the other hand, such chemical sensors should also be usable outside their nominal measuring ranges for gas concentration measurements.
Them are several known publications dealing with the problem of staying within a nominal measuring range in detection systems with sensors. To this end, a variable dilution of the measured sample medium is suggested. Among those known is a polycyclic hydrocarbon analyzer for the real time measurement of combustion aerosols, called PAS 1000e, by Seefelder MeBtechnik GmbH & Co. Vertriebs-KG, which is used for air quality monitoring. In order to stay within the measuring range of the photoelectric sensor used in this device, the measured aerosol must be diluted. The dilution is adjusted manually via a dial switch. The dilution of the measured aerosol ensures a safe measurement in the linear range of the photoelectric sensor. [East German] Patent publications DD 285 650 A5, DD 285 651 A5 and DD 285 652 A5, in contrast, describe a dynamic ratio for diluting a measuring medium in order to stay within the measuring range of a detection system. The processes described in these three publications assume that manual adjustment of the dilution in order to stay within the measuring range is questionable. For this reason, a feedback control system is described for a process-oriented readjustment of the dilution. The feedback control system provides a dynamic readjustment of the dilution ratio by evaluating the detection signal of the sensor in such a way that the mixing concentration that can be detected by the sensor is kept within the optimum--and thus nominal--measuring range of the sensor. The actual measuring result is determined by evaluating the detection signal of the sensor while considering the dilution ratio. The measuring medium is always a fluid. The adjusted dilution ratio is determined in each case by a volumetric flow rate measurement. This is accomplished with a dilution arrangement which dilutes the measured fluid medium, e.g., in nine dilution steps, from a dilution ratio of 1:256 in a dual reduction sequence to a dilution ratio of 1:1.
The problem with this volumetric flow rate measurement is the relatively complicated equipment. For example, it is required on the one hand, (see DD 285 651 A5) that metering vessels with a fixed volume be provided, which must also have a fixed ratio relative to each other. To achieve defined mixing ratios, these metering vessels must be interconnected with processor control. The line pieces necessary between the metering vessels and the associated valves must also be taken into consideration. On the other hand (see DD 285 652 A5), the volumetric flow rate measurement also requires a pump arrangement and hoses for transporting the measured medium, as well as means for changing the flow cross-section of the hoses so that the respective fluid volume can be determined by means of the hoses according to the Hagen-Poiseuille law.
The task underlying the invention consists of describing a process and a device for measuring a gas medium with at least one chemical sensor, whose nominal measuring range is safely complied with, even in the case of gas medium concentrations outside this nominal measuring range, and that is easily realized.